It could be a bad seam on the tank then.
As far as the servos go I prefer JR or Hitec. Futaba makes good servos but Ive had better luck with the other two. The JR Z650m would work well for both the steering and the throttle servo. Its costs a little more then the Futaba you were looking at but its a better servo. And definitely go with metal gears. It will save you in the long run.